Investigators have developed a variety of technical approaches to the generation of coordinate pair signals from electrographic devices. Industrial requirements for these devices are increasing concomitantly with the evolution of computer graphics, computer aided design and computer aided manufacturing systems. For such utilization, however, the digitizers or graphic tablets constituting such electrographic devices are called upon to perform in an electrically noisy environment. These environments have been observed to generate interfering frequencies, for example, from the synchronizing signals and the like of the visual readout components and of related electronics which necessarily are located in the region of operation of the tablets.
The operation of a digitizer or graphics tablet generally involves the same manual procedures as are employed in conventional graphics design, a stylus or tracer representing a writing instrument being drawn across or selectively positioned upon the digitizer surface. In turn, the electrographic device responds to the position of the stylus to generate analog paired coordinate signals which are digitized and conveyed to a host computer facility.
For the most part, digitizers have been fashioned as composite structures wherein a grid formed of two spaced arrays of mutually orthogonally disposed fine wires is embedded in an insulative carrier. One surface of this structure serves to yieldably receive a stylus input which is converted to coordinate signals. Various methods have been devised for generating coordinate defining signals, as a stylus-grid interaction, for example, a magnetostrictive effect may be established between stylus and grid or a capactive coupling effect may be evoked between these components.
The use of such grid structures, while providing accurate, linear output coordinate signals, necessarily involves intricate structures which are expensive to fabricate and are prone to damage in the normal course of use. Further, for many applications, it is desirable that the digitizer be fabricated as a highly transparent composite sheet. However, grid formations within the composite structures generally preclude such a transparency feature.
Early investigators have observed the advantage of developing digitizers having writing surfaces formed of a continuous resistive material coating. An immediately recognized advantage for this approach to digitizer design resides in the inherent simplicity of merely providing a resistive surface upon a supportive insulative subtrate such as glass or plastic. Further, the substrates and associated resistive coatings may be transparent to permit an expanded range of industrial applications.
The history of development of such resistive coating type devices shows that investigators have encountered a variety of technical problems, one of which being the non-uniform nature of the coordinate readouts achieved with the surfaces. Generally, precise one-to-one correspondence or linearity is required between the actual stylus or tracer position and the resultant coordinate signals. Because the resistive coatings cannot be practically developed without local resistance variations, for example of about +10%, the non-linear aspects of the otherwise promising design approach have impeded the development of practical devices until recently. However, certain important technical approaches to uitilizing the resistive surfaces have been achieved. For example, Turner discloses a border treatment or switching technique in U.S. Pat. No. 3,699,439 entitled "Electrical Probe-Position Responsive Apparatus and Method" issued Oct. 17, 1972, assigned in common herewith. This approach utilizes a direct current form of input to the resistive surface from a hand-held stylus, the tip of which is physically applied to the resistive surface. Schlosser et al. describe still another improvement wherein an a.c. input signal is utilized in conjunction with the devices and signal treatment of the resulting coordinate pair output signal is considerably improved. See U.S. Pat. No. 4,456,787 entitled "Electrographic System and Method", issued June 26, 1984, also assigned in common herewith. Position responsive performance of the resistive layer devices further has been improved by a voltage waveform zero crossing approach and an arrangement wherein a.c. signals are applied to the resistive layer itself to be detected by a stylus or tracer as described in U.S. Pat. No. 4,055,726 by Turner et al. entitled "Electrical Position Resolving by Zero-Crossing Delay" issued Oct. 25, 1977, and also assigned in common herewith.
As the designs of resistive layer surface digitizers now reach a level of technical development permitting their practical implementation, further need has been exhibited for their additional refinement with respect to improvements in linearity, i.e. with respect to the accuracy of their performance. While such refinements may be contemplated utilizing computer programming or software approaches, computational techniques generally have been observed to require software architecture evidencing such complexity as to render digital treatment too slow and to require overly expensive microprocessor devices.